Pentaetythritol is a kind of polyalcohol, widely used in synthetic resin, plasticizer, lubricating oil, explosive, and so on. It is suitable for use in coating furniture and walls for its good dumb light property. There is a large and increasing demand, especially for high quality (concentration is more than 98%) pentaerythritol. Abroad companies have monopolized the product in this field.
At present methods for preparing pentaerythritol is: reacting formaldehyde with acetaldehyde under alkaline condition into pentaerythritol and formate corresponding to the base, isolating and purifying further. The greatest defect of the method is the production of a large amount of byproducts, and the reaction condition is hard to control. For example, the more the formaldehyde is used compared with acetaldehyde, the more the methylal from pentaerythritol is created; when the amount of formaldehyde is reduced compare with acetaldehyde, the amount of byproducts such as dipentaerythritol, tripentaerythritol and polypentaerythritol is increased. Also, under alkaline condition, the redox reaction will take place between formaldehydes, thereby producing methanol and formate, and the higher the reaction temperature or the concentration of formaldehyde, and the more byproducts are produced. If the reaction temperature is too high, formaldehyde will be glycating to form glucide. Reactions also take place between acetaldehydes and cause the formation of impurities such as β-hydroxyl butyraldehyde.
The reaction of pentaerythritol is under study in the world. For example, UK Patent GB958654 explains the impact on the reaction caused by different reaction temperatures, reaction times and excess formaldehyde. It also discloses a conjunction method for several steps. China Patent CN 1165804A discloses a batch feeding manner, which controls impurities by varying the feeding rate into the reactor. As a result the yield of monopentaerythritol according to this patent is increased compared with continuous production, but the objections are also obvious: (1) reaction temperature is hard to control due to reaction heat concentrated in latter stage when performing the reaction in two stages, which results in partial overheating so as to increase side reactions; (2) yield of monopentaerythritol is decreased due to reacting in one-stage process with adding a mount of acetaldehyde to reduce the ratio of formaldehyde/acetaldehyde; (3) energy consumption is increased by refrigeration to keep low temperature in the first reaction stage to avoid the volatilization loss of acetaldehyde caused by reverse reaction of intermediate from acetaldehyde and formaldehyde. Another China Patent CN1097004 also discloses a condensation process with high-low temperature and low reactants ratio, and an isolation method to obtain monopentaerythritol, dipentaerythritol and tripentaerythritol by liquid accelerator. However, it is hard to obtain high quality monopentaerythritol, dipentaerythritol and tripentaerythritol products from this patent for the same reasons discussed above with respect to CN1165804A.